Move ImageDownloader out of the desktop widgets

This required a bit more untangling, but with this it seems we can build
subsurface-mobile again (at least on the desktop).

Interesting is the removal from inside the ImageDownloader of the call to
DivePictureModel::instance()->updateDivePictures() - which actually could
cause some interesting recursion issues. If it turns out we did indeed
need this, it needs to be re-architected.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface-core/CMakeLists.txt

@@ -76,6 +76,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	metrics.cpp
 	color.cpp
 	pluginmanager.cpp
+	imagedownloader.cpp
 	${SERIAL_FTDI}
 	${PLATFORM_SRC}
 	${BT_CORE_SRC_FILES}